1. Field of the Invention
The present invention relates to a liquid crystal panel display device used in a notebook personal computer or the like.
2. Description of the Prior Art
It is well known that notebook personal computers (referred to simply as "notebook computers" hereinafter) and laptop wordprocessors use nearly rectangular liquid crystal display devices as lids. A liquid crystal panel module is incorporated inside the liquid crystal display device, which consists of a light transmission liquid crystal panel unit and a backlight unit illuminating the liquid crystal panel unit from the rear. The backlight unit is constituted mainly of a light guide plate for guiding illumination light to the liquid crystal panel unit and a lamp arranged at one side of the light guide plate.
Brightness of a liquid crystal display at the liquid crystal panel display device constituted as above is adjusted through the amount of the illumination light at the backlight unit. Electrical power supplied to the lamp in the backlight unit is increased if the brightness of a screen of the liquid crystal display is to be increased.
A TFT color liquid crystal panel used in a color liquid crystal display device nowadays has a light transmittance of as low as approximately 6-10%. The transmittance is determined almost by a multiplication of an aperture ratio (50-80%) of a TFT array substrate, transmittance (30%) of a color filter and transmittance of a polarizing plate set at a bottom surface of the liquid crystal display panel. The transmittance of the polarizing plate of the conventional device is low, specifically, approximately 40%. As such, a relatively large luminance is required for the lamp to illuminate the whole face of the TFT color liquid crystal display panel brightly, which necessitates a large power to be fed to the lamp.
For instance, while the 12.1-inch TFT color liquid crystal panel used in a notebook computer requires nearly 2W power for the backlight unit, this power is about 1/3 of the total power fed to the notebook computer.
In the case where the notebook computer is driven by a battery, it is preferable that the consumption of power in the backlight unit be small, and for reducing the power consumed in the backlight unit, it is necessary to increase the transmittance of the liquid crystal panel.
The liquid crystal panel module employed in the conventional liquid crystal panel display device will be described hereinbelow.
FIG. 12 is a sectional view of a nearly rectangular liquid crystal panel module 100 used in a liquid crystal panel display device in the prior art. The liquid crystal panel module 100 consists of a backlight unit of nearly the same shape, i.e., nearly rectangular, including a lamp 101, a reflector 102 and a light guide plate 106, and a liquid crystal panel unit of almost the same shape, namely, nearly rectangular having polarizing plates 103, 105 attached to front and rear faces of a liquid crystal panel 104. A reflecting sheet 107 is attached to a bottom surface of the light guide plate 106. A plurality of sawtoothed diffused reflection parts 108a-108d are provided on the reflecting sheet 107 to diffuse and reflect the light of the lamp 101.
The polarizing plate 105 attached to the rear face of the liquid crystal panel 104 passes only light of a predetermined polarization direction, and absorbs light in other polarization directions. The polarizing plate 103 attached to the front face of the liquid crystal panel 104 passes only light in a direction parallel to or perpendicular to a polarization axis of the polarizing plate 105, absorbing light of the other polarization directions.
In the above-described constitution, circularly polarized light, including horizontal oscillation waves (p polarized light) and perpendicular oscillation waves (s polarized light) with respect to the bottom face of the light guide plate 106 are brought into the light guide plate 106 of the liquid crystal panel module 100. No less than half of the lamp light is absorbed at the polarizing plate 105. Since the light is absorbed also within the light guide plate 106, the transmittance of light is decreased to as low as about 40% as discussed above.
FIG. 13 is a diagram showing the constitution of a liquid crystal panel module 200 proposed to solve the aforementioned problem. The liquid crystal panel module 200 comprises a backlight unit of a lamp 201, a reflector 202 a light guide plate 206 and, a liquid crystal panel unit of a TFT color liquid crystal panel 204 held by a polarizing plate 203 and a polarizing separating film 205. The polarizing separating film 205 is a polarizing film of a reflecting type, for example, DBEF by Sumitomo 3M Co., Ltd. (registered trade name by Sumitomo 3M), passing light in a predetermined polarization direction and reflecting light in the other polarization directions. The polarizing plate 203 allows only light in a polarization direction parallel or perpendicular to a polarization axis of the polarizing separating film 205, whole absorbing light in the other polarization directions. Similarly to the liquid crystal panel module 100, a reflecting sheet 207 is attached to a bottom surface of the light guide plate 206, with a plurality of sawtoothed diffused reflection parts 208a-208d to diffuse and reflect the light from the lamp 201.
In the thus-constituted liquid crystal panel module 200, the light that cannot pass through the polarizing separating film 205 is reflected totally to the light guide plate 206. The light reflected towards the light guide plate 206 is changed to be irregular in polarization direction at the diffused reflection part 208 at the lower surface of the light guide plate 206, and then sent to the polarizing separating film 205 again. The polarizing separating film 205 passes, among the light input thereto, light in a predetermined polarization direction generated when the light is changed in the polarization direction at the diffused reflection part 208, and reflects light in the other polarization directions. This reflection of light is repeatedly carried out in the light guide plate 206, whereby the amount of light passing the polarizing separating film 205 is increased, thereby decreasing loss of the lamp light.
However, the polarizing separating film 205 is complicated in structure and expensive as compared with general polarizing films, hence inviting a cost rise of the liquid crystal panel module 200.